Understanding Java Servlet Architecture: UML, Execution Process, Roles, and Lifecycle

In Java Web development, Servlets serve as a fundamental component; mastering them provides a solid foundation for advanced frameworks such as SSH, SSM, and micro‑service architectures.

UML Overview

The Servlet class hierarchy consists of the abstract class HttpServlet extending GenericServlet, which implements the Servlet, ServletConfig, and Serializable interfaces. User‑defined servlets (e.g., MyServlet) extend HttpServlet and override doGet() and doPost().

Execution Process in a Servlet Container

A request from a browser (e.g., a GET request) is received by the container, which creates HttpServletRequest and HttpServletResponse objects, locates the target servlet, spawns a thread, and invokes the servlet’s service() method. Depending on the request type, service() delegates to doGet() or doPost(). After processing, the thread is returned to the pool or destroyed.

Key notes: each servlet typically has a single instance; each request is handled by a separate thread; multiple threads may operate on the same servlet, which can lead to thread‑safety issues.

Servlet’s Role in JavaWeb

Servlets act both as pages and as controllers. With the advent of JSP and other view technologies, the controller role becomes dominant, forming the classic MVC three‑layer architecture (JSP + Servlet + Model).

Example of a doGet() implementation:

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    request.setCharacterEncoding("UTF-8");
    response.setContentType("text/html;charset=utf-8");
    PrintWriter out = response.getWriter();
    out.println("Hello!Servlet.");
}

Servlet Lifecycle

The lifecycle consists of loading the servlet class, instantiating the servlet (executing the no‑arg constructor), invoking init() once, handling requests via service() (which calls doGet() or doPost()), and finally calling destroy() before the servlet is removed.

Understanding these concepts equips developers with the knowledge needed to build robust, scalable backend services using Java Servlets.